The accurate operation of many integrated circuits, such as analog-to-digital converters, depends upon the ability of these circuits to reference a constant voltage. Therefore, any factors affecting the stability of the reference voltage would have an adverse effect upon the proper operation of these circuits. Since temperature variations are a common cause of voltage fluctuations in electrical circuits, there is therefore a need for a circuit that provides a substantially temperature-stable reference voltage.
The temperature stability of a bandgap reference voltage is commonly known in the art. A discussion of bandgap reference voltage circuits is found in "Analysis and Design of Analog Integrated Circuits" by P. R. Gray et al, John Wiley & Sons, 1977, at pages 254-261. Basically, a bandgap reference voltage, V.sub.REF, is provided by a weighted sum, V.sub.REF =aV.sub.BE +bV.sub.T, where V.sub.BE is the base-emitter junction voltage of a bipolar junction transistor, and V.sub.T is the thermal voltage kT/q. Typically, V.sub.T is obtained from the difference, .DELTA.V.sub.BE, between the base-emitter junction voltages of two bipolar transistors. Because the temperature coefficients of V.sub.BE and V.sub.T have opposite signs, therefore, with proper weighting factors a and b, the sum aV.sub.BE +bV.sub.T, or the sum aV.sub.BE +c.DELTA.V.sub.BE, can theoretically be adjusted to have a zero temperature coefficient.
Due to the increasing use of CMOS integrated circuits, there has been a desire for a bandgap reference voltage circuit formed by CMOS processes. One such CMOS bandgap reference voltage circuit is disclosed in U.S. Pat. No. 4,588,941, issued to D. A. KERTH on May 13, 1986. Another CMOS bandgap voltage reference circuit is described in "Precision Curvature--Compensated CMOS bandgap reference" by B. Song et al in IEEE Journal of Solid State Circuits, Volume SC-18 No. 6, December 1983, pages 634-643.
The CMOS bandgap reference voltage circuits disclosed by the above-cited references are considered undesirable for several reasons. One of the disadvantages of these prior art circuits is their use of operational amplifiers, which usually increase the complexity and, as a result, the die size, of the circuits. Thus, one object of this invention is to have a circuit whereby a bandgap reference voltage is provided without the use of operational amplifiers.
Also, when implemented by P-well CMOS technology, the prior art circuits typically provide the reference voltage with respect to a power supply voltage (for example, V.sub.DD) rather than with respect to ground. Because power supply voltages are more susceptible to noise, therefore another object of this invention is to have a CMOS circuit whereby the bandgap reference voltage can be provided with respect to ground.